The general electric equipments that need a great amount electric power such as industrial computers or computers used on aircrafts require a steady and reliable power supply due to they have to continuously process huge and complex information. To prevent abrupt loss of power supply or instant pulse resulting from switching to backup power that might damage the expensive electric equipments or cause instant loss of information under processing, a N+M redundant power supply system is generally adopted to maintain regular operation without interrupting the power supply. N represents the number of power supply devices assembled to meet total power loading requirement of the electric equipments. M represents the allowable number of power supply devices that can be out of service. N≧1 and M≧1. In the redundant power supply system that consists of N+M sets of power supply devices, each power supply device includes a stationary power unit to supply DC power constantly and a main power unit which outputs DC power in a power-ON condition. Many present redundant power supply systems also provide improved backup or fault tolerance function. For instance, U.S. patent application Ser. No. 11/022,795 discloses a redundant power supply having AC/DC input. Such a design of dual power sources input allows the DC power to be continuously delivered to electric equipments to maintain regular operation when any of the power input sources is interrupted.
The redundant power supply system mentioned above has main power units electrically coupled in a parallel fashion. Take a 1+1 redundant power supply system as an example. Theoretically, two main power units are set to evenly share the loading current to drive the electric equipments to operate regularly. But in practice, any of the main power units could be aged or encounter variation of output impedance that cause even sharing of the power not possible. To force them to evenly supply the same amount of power could cause overload of the main power unit. To maintain the regular operation of the electric equipments, another main power unit has to bear the total output power and becomes overloaded. This greatly shortens the life span of the redundant power supply system. To remedy this problem, a current sharing technique has been developed for the main power units. For instance, U.S. patent publication No. 20060273740 discloses such a technique. It provides an apparatus to detect output DC power and generate a feedback signal to the main power unit to regulate output power. Hence when an abnormal condition of output power of the main power unit is detected, the DC power to be output is controlled while another main power unit supplies a greater amount of DC power. As a result the main power units can be regulated to provide optimal power output. The life span of the redundant power supply system increases and damage resulting from abrupt conditions can be prevented. U.S. patent application Ser. No. 11/330,079 also discloses a Backup-type power supply system in which output power provided by the power supply system is integrated by different power supply modules according to different power levels. Each power supply device has an independent power balance unit to balance output power to achieve optimal power output at each power level. However, the balance mode for the output power set forth above mainly targets the main power units.
In addition to the main power units, the stationary power units in the redundant power supply system have to provide stationary power for the electric equipment during start and stop in the regular condition. As the electric equipments become more complex and require increasing amount of power, demand on the stationary power unit also increases. Moreover, lots of electric equipment these days adopts a technique to let the stationary power unit to function independently during the stop condition to support external electronic devices, such as lighting devices, small LCD devices and the like. All this application is performed without power ON of the electric equipments. As a result, the stationary power units also might encounter the condition of unbalanced power supply happened to the main power units previously discussed, and a forced and overloaded output power supply could shorten the life span of the standby power units. Power supply in the regular condition could also be impacted. Overload of the stationary power units could also result in abrupt power interruption. And the electronic elements in the electric equipments that rely on the power of the stationary power units could lose voltage instantly and be damaged.